This invention generally relates to the process of forming a colored oxide coating on an aluminum workpiece wherein the aluminum workpiece is first anodized to form a porous oxide coating and then is subjected to electrolysis in an aqueous bath containing coloring agents which are deposited into the porous coating during electrolysis. The first process of this general type to be commercially used to any significant extent was the process described by Asada in U.S. Pat. No. 3,382,160. In this process the aluminum workpiece is first anodized in an aqueous sulfuric acid electrolyte to form a porous anodic oxide coating and then subsequently electrolytically treated in an acidic aqueous bath containing metal salts such as the soluble salts of nickel, cobalt, iron and the like to generate the color by precipitating metal from solution into the porous oxide coating. The more metal that is incorporated into the oxide layer, the darker the anodic coating becomes.
Many modifications to this basic process have been made over the years which include adding various metallic salts, boric acid and magnesium sulfate to the electrolytic bath. Both alternating and direct currents have been employed. The basic process has been widely used because it has been found to be less costly to operate than color anodizing processes wherein the color is generated within the anodic oxide coating as the coating is formed in the anodizing process. Other references which typically illustrate the state of the prior art relating to the basic process include U.S. Pat. Nos. 4,251,330 (Sheasby), 3,616,309 (Asada et al) 3,674,563 (Asada) and 3,788,956 (Patrie et al).
Although successful, this process generally has significant color control problems from the standpoint of generating a uniform color across the surface of the workpiece and from the standpoint of matching the color of workpieces which are sequentially treated in the same bath and workpieces which are electrolytically treated in separate baths. These difficulties were in large part caused by the poor throwing power of the electrolyte particularly when producing the darker colors. For example, when workpieces having large planar surfaces are subjected to electrolysis the edges of the workpiece tend to be much darker than the center sections, which is commonly termed "window frame" effect. Additionally, when workpieces having complex shapes are electrolytically treated the portions of the workpiece shielded from the counter electrode tend to be incompletely coated and thus develop a much lighter color than the remainder of the workpiece.
Additionally, in many instances when electrolyzing an anodized aluminum workpiece in accordance with the basic process, the anodic oxide coating tends to spall and break away from the aluminum substrate due to the disruptive effects of the electrolyzing current on the bond between the anodic coating and the substrate. This was believed to be due in part to the effects of sodium in the electrolyte and to minimize this effect, large quantities of magnesium sulfate were frequently added to the electrolytic bath.
It is against this background the present invention was developed.